D
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Factors affecting transfer flow: Transfer area, engine displacement, crankcase pressure.
Here's the formula for flow velocity:
Velocity (m/s) = Volume flow (m3/s) / area (m2)
Air/fuel velocity through transfer ports increases with greater air/fuel volume flow thru the same port area (which happens when you modify the engine with a larger bore), and decreases with greater port area to channel the same air/fuel volume (which happens when you enlarge the transfers or add boost ports).
Here's my theory on how air/fuel velocity change is related to powerband change. First off, increasing the crankcase compression ratio (CCR) increases the pressure that exists in the crankcase right before the transfers open. More pressure increases the volume flow and thus the velocity. So there are 3 things that can increase transfer velocity; enlarging the cylinder bore, reducing the transfer port area, and increasing CCR.
Why is velocity important? More transfer speed means the air/fuel charge will reach the spark plug faster. You want the flow speed to match the type powerband you want. The scenario most likely is that the end of the transfer charge arrives at the spark plug (when it fires) at the lower rpm of the powerband, and the beginning of the charge is at the spark plug at the upper rpm of the powerband. You can slow down that arrival time by changing the transfer ports roofs to be more level. That directs the flow so that there is more of a horizontal vector and less of a vertical. More of the flow energy is dissipated in the left and right flow collision with more horizontal expansion of the combined charge.
What is interesting is that small engines need less percentage of the cylinder bore circumference than larger engines. Here's an example to prove it: A 45mm bore and stroke engine has 1590mm bore area. The circumference is 141mm which is 9% of the 1590. Whereas a 90mm bore/stroke engine has 6361mm bore area. The circumference is 283 which is 4.4% of it. So if the two engines have the same percentage of circumference for transfer port horizontal opening the 45mm bore engine has twice the flow area. If they have the same percentage of the circumference (say 50%) then that means the small bore engine needs less CCR to achieve the same transfer flow velocity. Maintaining the same CCR allows the small engine to be more adept at high rpm running than larger engines.
I have always been perplexed at why the designers of my small cylinders (48cc, 55cc, 60cc) have only one transfer on each cylinder side and utilize a low CCR. Having narrow transfers strikes a balance with the low CCR. Both can be small just because it's a small engine which means its transfers are automatically more efficient.
Boost ports
Since the flow from boost ports is mostly vertical with no collisions then it arrives at the spark plug even faster than the transfer flow. I had experimented with boost port size and determined 30% of the transfer port is ideal. How some engines get away with huge transfers is beyond me. Maybe that is just a fad, and in reality the engines could run stronger if their boost ports were narrowed.
Here's the formula for flow velocity:
Velocity (m/s) = Volume flow (m3/s) / area (m2)
Air/fuel velocity through transfer ports increases with greater air/fuel volume flow thru the same port area (which happens when you modify the engine with a larger bore), and decreases with greater port area to channel the same air/fuel volume (which happens when you enlarge the transfers or add boost ports).
Here's my theory on how air/fuel velocity change is related to powerband change. First off, increasing the crankcase compression ratio (CCR) increases the pressure that exists in the crankcase right before the transfers open. More pressure increases the volume flow and thus the velocity. So there are 3 things that can increase transfer velocity; enlarging the cylinder bore, reducing the transfer port area, and increasing CCR.
Why is velocity important? More transfer speed means the air/fuel charge will reach the spark plug faster. You want the flow speed to match the type powerband you want. The scenario most likely is that the end of the transfer charge arrives at the spark plug (when it fires) at the lower rpm of the powerband, and the beginning of the charge is at the spark plug at the upper rpm of the powerband. You can slow down that arrival time by changing the transfer ports roofs to be more level. That directs the flow so that there is more of a horizontal vector and less of a vertical. More of the flow energy is dissipated in the left and right flow collision with more horizontal expansion of the combined charge.
What is interesting is that small engines need less percentage of the cylinder bore circumference than larger engines. Here's an example to prove it: A 45mm bore and stroke engine has 1590mm bore area. The circumference is 141mm which is 9% of the 1590. Whereas a 90mm bore/stroke engine has 6361mm bore area. The circumference is 283 which is 4.4% of it. So if the two engines have the same percentage of circumference for transfer port horizontal opening the 45mm bore engine has twice the flow area. If they have the same percentage of the circumference (say 50%) then that means the small bore engine needs less CCR to achieve the same transfer flow velocity. Maintaining the same CCR allows the small engine to be more adept at high rpm running than larger engines.
I have always been perplexed at why the designers of my small cylinders (48cc, 55cc, 60cc) have only one transfer on each cylinder side and utilize a low CCR. Having narrow transfers strikes a balance with the low CCR. Both can be small just because it's a small engine which means its transfers are automatically more efficient.
Boost ports
Since the flow from boost ports is mostly vertical with no collisions then it arrives at the spark plug even faster than the transfer flow. I had experimented with boost port size and determined 30% of the transfer port is ideal. How some engines get away with huge transfers is beyond me. Maybe that is just a fad, and in reality the engines could run stronger if their boost ports were narrowed.
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